1. Field of the Invention
The present invention relates to the art of papermaking. More specifically, the present invention relates to the art of papermachine dryer regulation by means of automatic data process and control devices.
2. Description of the Prior Art
From an overall, simplified perspective, the manufacture of paper from wood fiber is a drying process. Prepared stock comprising a dilute aqueous slurry of wood fiber is directed onto a traveling screen for an initial, gross separation of water from fiber. As the water flows through the screen openings, constituent fiber is accumulated and retained on the screen surface to form a wet, fibrous mat. Additional water is subsequently removed from the mat by mechanical pressing.
Screening and pressing steps remove approximately 96% of the water initially present in the original slurry leaving a consolidated paper web containing approximately 63% water and 37% dry fiber. Since a satisfactory finished paper web should contain approximately only 5% water in relation to the dry fiber weight, such additional water removal is normally accomplished by means of thermal vaporization. For this purpose, the web is passed in intimate surface contact over a successive series of steam heated, rotating cylinders, such web being pressed against the hot surface of each cylinder about a major portion of the circumferential arc by an overlying web of woven fabric.
Contemporary papermachine design practice divides the 70 to 100 cylinders of the drying portion of a papermachine into three or more sections for the purpose of steam distribution and management. For reasons to be explained, the final steam section of the dryer sequence relative to the paper web progression is provided the highest temperature steam and the greater proportionate share of the available steam energy. As the steam flow progresses counterflow of the web travel through the several sections of the machine dryers toward the wet press end, dryer cylinder surface temperature decreases. The control mechanics of such temperature management is by means of pressure differential regulation across the several steam distribution sections of the dryer line. Utilization efficiency of the available steam energy is, of course, the objective of such pressure and temperature management strategy but the rational support of such strategy relates to the micro-mechanics of the web drying process.
Since the web is extremely wide in relation to the thickness thereof, only the wide surfaces are available for water vapor transpiration from the web envelope to the surrounding atmosphere. Upon reaching thermodynamic drive conditions relative to the surrounding atmosphere, water present at or near the surface of a saturated web is vaporized first, leaving interstitial capacity at the web surface to receive, by capillary migration, additional water from the web interior. Under constant (relative to time) thermodynamic drive conditions, the aforedescribed vaporizing mechanism will progress at an approximately constant rate in terms of water mass removal per unit of time and surface area. After this drying process has progressed a certain degree toward completion, however, the rate of water removal begins to diminish. The water content of the web at this point of removal rate diminution is characterized as the critical moisture content. If a lower final moisture content of the web is desired, the thermodynamic drive conditions must be intensified. Hence, the need for higher pressure, higher temperature steam in the later portion of the dryer line.
Although the driest portion of the web receives the greatest magnitude of thermal energy, the return from such expenditure of energy in terms of moisture removed diminishes exponentially toward the dry end of the dryer line. Consequently, upon reaching the critical moisture content, the web drying rate thereafter is described as "falling."
In summary then, the web enters the dryer line at approximately 160% to 170% moisture content, experiences constant rate drying in terms of moisture removal per unit of time until reaching the critical moisture content in the order of 35% to 45% and is completed at a falling drying rate.
Two of the several factors affecting the magnitude of critical moisture content are the intensity of constant rate drying and the pulp stock drainage rate.
Drying intensity describes the magnitude of the thermodynamic drive in terms of water mass removed per unit of time. Under more intense thermodynamic conditions, water is removed more rapidly but consequently arrives at a greater critical moisture content. Under extreme conditions, a circumstance characterized as "case hardening," may be removed so rapidly from the web surface as to drive the web surface elements to such a low moisture content as to inhibit the transmission of sufficient heat to vaporize moisture retained at the web center.
Stock slowness is another factor affecting critical moisture content. The term slowness describes the time required for given quantity of water to drain from a stock sample. The same characteristic is more generally termed as drainage rate. Although raw pulp has a substantial drainage rate, the characteristic of slowness is further developed or increased as a consequence of refining which is applied for the primary purpose of web strength development. Relative to drying, it has been found that stock slowness affects the magnitude of critical moisture content in the relation that a slower stock will reach a lower characteristic critical moisture content, all other factors remaining constant.
It may be concluded from the foregoing that for a given stock (slowness) laid to a given thickness (basis weight) on a given papermachine (number and configuration of drying cylinders) there is an optimum drying rate to most efficiently utilize available steam in arriving at a target end moisture content. Such optimum conditions may be tailored to consume the least amount of steam for a given production rate or to elicit the greatest possible product (machine speed) from the magnitude of steam energy available. In either case, for a given stock and web thickness on a given machine, there is an optimum production efficiency in terms of paper production quantity per unit of steam or heat energy consumed.
Although most of the foregoing theoretical or conceptual precepts are well known to the prior art of papermaking, the specific application of these precepts to a particular papermachine, running a particular but variable pulp requires considerably more finesse than science.
Normally, papermachine dryer control is a fixed, pressure differential regulation between the several dryer sections. If the machine is dryer-limited, i.e. set for exploiting all the steam available from generation sources, control is simply a matter of speed regulation. The machine speed limit is set against the moisture content of the web at the reel. U.S. Pat. No. 3,801,426 includes a representative disclosure of this type of control.
If the machine is not dryer limited so that the machine speed is determined by other factors and sufficient excess steam capacity is available to dry as much web as the machine will otherwise produce, control takes the form of active pressure regulation. U.S. Pat. No. 3,930,934 is a representative disclosure wherein appropriate sensor signals of web basis weight, moisture and temperature characteristics are processed by automatic data processing equipment with historically developed computer programs for the purpose of actively regulating the steam supply pressure (and hence, the dryer, temperatures). Abundantly available steam energy allows the web to follow a consequential drying rate trajectory which may or may not be the most energy efficient trajectory for the particular stock furnish from which the web is laid.
It is an objective of the present invention, therefore, to teach a papermachine dryer control method and apparatus whereby the pulp characteristic of drainage rate is a pivotal control variable in the determination of an optimum, energy efficient paper web drying trajectory.
Another objective of the present invention is to provide a feed-forward type of control system for active regulation of steam pressure differential regulation between the several dryer sections of a papermachine.
Another objective of the present invention is to teach a method for drying a particular paper web to the lowest possible moisture content in the constant rate phase thereby minimizing steam requirements for the falling rate phase.
Another objective of the present invention is to teach a method of deriving the greatest possible production rate from a given papermachine running a particular stock with dryer limited steam capacity.